1. Field of the Invention
This invention relates to polymeric materials, more particularly it relates to compositions with three aspects: thermoset polymer, shape memory polymer (SMP) capable of shrinking at temperatures above the SMP's glass transition temperature, and thermoplastic polymer or other means for healing at a molecular level; which compositions generally have repeatable, self-healing properties without the need for external shape confinement.
2. Description of Related Art
Polymers are large molecules (macromolecules) composed of repeating structural units. These sub-units are typically connected by covalent chemical bonds. The term polymer encompasses a large class of compounds comprising both natural and synthetic materials with a wide variety of properties. Because of the extraordinary range of properties of polymeric materials, they play essential and ubiquitous roles in everyday life. These roles range from familiar synthetic plastics and elastomers to natural biopolymers such as nucleic acids and proteins that are essential for life.
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids that are moldable. Plastics are typically organic polymers of high molecular mass, but they often contain other substances. There are two types of plastics: thermoplastic polymers and thermosetting polymers.
Thermoplastics are the plastics that do not undergo chemical change in their composition when heated and can be molded again and again. Examples include polyethylene, polypropylene, polystyrene, polyvinyl chloride, and polytetrafluoroethylene (PTFE). Common thermoplastics range from 20,000 to 500,000 amu.
In contrast, thermosets are assumed to have large molecular weight. These chains are made up of many repeating molecular units, known as repeat units, derived from monomers; each polymer chain will have several thousand repeating units. Thermosets can take shape once; after they have solidified, they stay solid. In the thermosetting process, a chemical reaction occurs that is irreversible. According to an IUPAC-recommended definition, a thermosetting polymer is a prepolymer in a soft solid or viscous state that changes irreversibly into an infusible, insoluble polymer network by curing. The cure may be done through heat (generally above 200° C. (392° F.)), through a chemical reaction (two-part epoxy, for example), or irradiation such as electron beam processing. A cured thermosetting polymer is often called a thermoset.
Thermoset materials are usually liquid or malleable prior to curing and designed to be molded into their final form, or used as adhesives. Others are solids like that of the molding compound used in semiconductors and integrated circuits (IC). In contrast to thermoplastic polymers, once hardened a thermoset resin cannot be reheated and melted back to a liquid form.
The curing process transforms the thermosetting resin into a plastic or rubber by a cross-linking process. Energy and/or catalysts are added that cause the molecular chains to react at chemically active sites (unsaturated or epoxy sites, for example), linking into a rigid, 3-D structure. The cross-linking process forms a molecule with a larger molecular weight, resulting in a material with a heightened melting point. During the curing reaction, the molecular weight has increased to a point so that the melting point is higher than the surrounding ambient temperature, the material forms into a solid material.
However, uncontrolled heating of the material results in reaching the decomposition temperature before the melting point is obtained. Thermosets never melt! Therefore, a thermoset material cannot be melted and re-shaped after it is cured. A consequence of this is that thermosets generally cannot be recycled, except as filler material.
Thermoset materials are generally stronger than thermoplastic materials due to their three dimensional network of bonds (cross-linking). Thermosets are also better suited to high-temperature applications (up to their decomposition temperature). However, they are more brittle. Because of their brittleness, thermoset is vulnerable to high strain rate loading such as impact damage. Since a lot of lightweight structures use fiber reinforced thermoset polymer composites, impact damage, if not healed properly and timely, may lead to catastrophic structural failure.
A thermoplastic material, also known as a thermosoftening plastic, is a polymer that turns to a viscous liquid when heated and freezes to a rigid state when cooled sufficiently. Most thermoplastics are high-molecular-weight polymers whose chains associate through weak Van der Waals forces (polyethylene); stronger dipole-dipole interactions and hydrogen bonding (nylon); or even stacking of aromatic rings (polystyrene). As noted herein, thermoplastic polymers differ from thermosetting polymers (e.g. phenolics, epoxies) in that they can be remelted and remolded.
Thermoplastics can go through melting/freezing cycles repeatedly and the fact that they can be reshaped upon reheating gives them their name. However, this very characteristic of reshapability also limits the applicability of thermoplastics for many industrial applications, because a thermoplastic material will begin to change shape upon being heated above its Tg and Tm.
Initiation of cracks and other types of damage on a microscopic level has been shown to change thermal, electrical, and acoustical properties, and eventually lead to whole scale failure of the material. From a macromolecular perspective, stress induced damage at the molecular level leads to larger scale damage called microcracks. A microcrack is formed where neighboring polymer chains have been damaged in close proximity, ultimately leading to the weakening of the polymer as a whole. In view of the diverse use of polymers in industry, it is self-evident that failure of safety-critical polymer components such as brittle thermoset polymers is a serious problem; failure of these materials can lead to serious even catastrophic accidents.
For thermoset polymers that have developed cracks, unfortunately there are only two fundamental choices, attempt to repair the crack or entirely remove and replace the component that contains the damaged material. Usually, cracks are mended by hand, which is difficult because cracks are often hard to detect. A polymeric material that can intrinsically correct damage caused by normal usage could lower production costs through longer part lifetime, reduction of inefficiency over time caused by degradation of the part, as well as prevent costs incurred by material failure.
In an attempt to heal damage, restore mechanical properties and extend the service life of polymers, the concept of crack healing in polymeric materials has been widely investigated [Jud K, Kausch H H. Polymer Bulletin 1979; 1:697-707; Keller M W, White S R, Sottos N R. Polymer 2008; 49: 3136-3145; Prager S, Tirrell M. J Chem Phys 1981; 75:5194-8; De Gennes P G. J Chem Phys 1971; 55:572-9; Doi M, Edwards S F. J Chem Soc: Faradays Trans 2 1978; 74:1789-1801; Kim Y H, Wool R P. Macromolecules 1983; 16:1115-1120; Kausch H H, Jud K. Rubber Process Appl 1982; 2:265-268; Wool P R, O'Connor K M. Polym Eng Sci 1981; 21:970-977; McGarel O J, Wool R P. J Polym Sci Part B—Polym Phys 1987; 25: 2541-2560; Yang F, Pitchumani R. Macromolecules 2002; 35:3213-3224; Chung C M, Roh Y S, Cho S Y, Kim J G. Chem Mater 2004; 16:3982-3984; Yuan Y C, Rong M Z, Zhang M Q, Yang G C. Polymer 2009; 50:5771-5781; Takeda K, Tanahashi M, Unno H. Sci Tech Adv Mater 2003; 4:435-444; Kalista S J, Ward T C. J R Soc Interface 2007; 4:405-411; Chipara M, Wooley K. Mater Res Soc Symp 2005; 851:127-132].
In thermoplastic polymers, the most widely studied and reported mechanism for self healing is the molecular inter-diffusion mechanism. It has been reported [Jud K, Kausch H H. Polymer Bulletin 1979; 1:697-707] that when two pieces of the same polymer are brought into contact at a temperature above its glass transition temperature (Tg), the interface gradually disappears and the mechanical strength at the polymer-polymer interface increases as the crack heals due to molecular diffusion across the interface. To better explain the process of crack healing by this mechanism, various models have been proposed [Keller M W, White S R, Sottos N R. Polymer 2008; 49: 3136-3145; Prager S, Tirrell M. J Chem Phys 1981; 75:5194-8; De Gennes P G. J Chem Phys 1971; 55:572-9; Wool R P, O'Connor K M. J Appl Phys 1981; 52:5953-5963]. In particular, Wool and O'Connor [Wool R P, O'Connor K M. J Appl Phys 1981; 52:5953-5963] suggested a five-stage model to explain the crack healing process in terms of surface rearrangement, surface approaching, wetting, diffusion and randomization. Kim and Wool [Kim Y H, Wool R P. Macromolecules 1983; 16:1115-1120] also presented a microscopic theory for the diffusion and randomization stages. In another study [Kausch H H, Jud K. Rubber Process Appl 1982; 2:265-268], it was observed that the development of the mechanical strength during the crack healing process of polymers is related to interdiffusion of the molecular chains and subsequent formation of molecular entanglements. Other reported healing mechanisms in thermoplastic polymers include photo induced healing, recombination of chain ends, self healing via reversible bond formation, and with nanoparticles [Wu D Y, Meure S, Solomon D. Prog Polym Sci 2008; 33:479-522].
In thermoset polymers, self-healing mechanisms by the incorporation of external healing agents such as liquid healing agent (monomer) encased in hollow fibers [Kirkby E L, Michaud V J, Månson J A E, Sottos N R, White S R. Polymer 2009; 50: 5533-5538; Blaiszik B J, Caruso M M, McIlroy D A, Moore J S, White S R, Sottos N R. Polymer 2009; 50: 990-997], micro-capsules [Brown E N, White S R, Sottos N R. Compos Sci Technol 2005; 65:2474-2480; Brown E N, White S R, Sottos N R. J Mater Sci 2006; 41:6266-6273], and solid healing agent (thermoplastic particles) dispersed in the thermoset matrix [Wu D Y, Meure S, Solomon D. Prog Polym Sci 2008; 33:479-522; Zako M, Takano N. J Intell Mater Syst Struct 1999; 10:836-841], have; been proposed and tested. However, the different physical and behavioral characteristics of thermoset SMPs relative to standard thermosets make the applicability or suitability of a component on one type of the thermosets of uncertain relevance to the any. For example, with regular thermosets suitability of an additional component depends on the chemical compatibility, viscosity of the molten thermoplastic, and the concentration gradient, whereas for thermoset SMPs, suitability of an additional component also depends on the diffusion under the recovery force.
Self-healing of polymer components has been a much-explored but essentially unmet need. Some polymers by themselves possess the self-healing capability such as thermally reversible crosslinked polymers [Chen X, Wudl F, Mal A K, Shen H, Nutt S R. Macromolecules 2003; 36:1802-1807] and ionomers [Varley R J, Shen S, van der Zwaag S. Polymer 2010; 51: 679-686]. Although these systems are very successful in healing micro-length scale damage, they face tremendous challenge when they are used to repair large, macroscopic, structural-length scale damage, which are visible by the naked eye [Li G and Nettles D. Polymer 2010; 51:755-762; Li G and Uppu N. Comp Sci Tech 2010; 70: 1419-1427; Nji J and Li G. Smart Mater Struct 2010; 19: paper No. 035007]. Prior to the present invention, the state-of-the-art for self-repair of thermosetting polymers and their composites includes: (1) use of hollow fibers/microcapsules to release polymeric resin when ruptured, and heal the crack through in-situ polymerization triggered by the catalyst contained in the polymer matrix; (2) use of thermoplastic particles to flow into the crack when heated up and glue the crack when cooled down; and (3) use of thermo-reversible covalent bonds via the retro Diels-Alder (DA) reaction. Despite the significant advancements made using a bio-mimetic approach, there is still a long way to go before even the simplest biological healing mechanism can be replicated within these synthetic materials. One immediate difference between biological and these synthetic healing mechanisms is that biological systems involve multiple-step healing solutions. For example, mammalian healing processes rely on fast forming patches to seal and protect damaged skin before the slow regeneration of the final repair tissue.
Several self-healing schemes have been reported in the literature primarily for healing microcracks, including incorporation of external healing agents such as liquid healing agent in microcapsules, hollow fibers, and microvascular networks, and solid healing agent such as embedded thermoplastic particles. Some polymers by themselves possess self-healing capabilities, including ionomers, which consist of over 15% of ionic groups, and a highly cross-linked polymer, which is synthesized via the Diels-Alder (DA) cycloaddition of furan and maleimide moieties, and the thermal reversibility of the chemical bonds is accomplished via the retro-DA reaction. A combination of microcapsule and shape memory alloy (SMA) wire has also been studied. Because damage is usually in structural-length scale, the challenge is how to heal macrocracks. However, the existing systems are unable to very effectively heal macroscopic damage. For instance, in order to heal macrocracks, a large amount of healing agent is needed. However, incorporation of a large amount of healing agent will significantly alter the physical/mechanical properties of the host structure. Also, large capsules/thick hollow fibers themselves may become potential defects when the encased healing agent is released. Therefore, the grand challenge facing the self-healing community is how to heal structural-length scale damage such as impact damage autonomously, repeatedly, efficiently, timely, and at the molecular-length scale.
One further problem with the existing self-healing systems is the presence of voids after the healing process. For example, a polymeric material will contain microcapsules of monomer throughout, and similarly an initiator/catalyst would be uniformly present throughout the material. When a crack occurs, the monomer-bearing capsules at the site of the crack would rupture, disgorge monomer and polymerization would result because of the presence of the initiator. Prior to polymerization, the capillary forces at the crack face would encourage even flow of monomer, resulting in an evenly-healed crack. However, after the crack has healed the material now has voids where the monomer capsules used to be. These voids can have an adverse effect upon the material's mechanical properties. Moreover, this self-healing process is available for only one time in the area of the healed crack.
Recently, shape memory polymer (SMP) has emerged as new type of smart material. Various types of applications have been studied, particularly in lightweight structure applications. Tey et. al [Tey S J, Huang W M and Sokolwski W M 2001. Influence of long term storage in cold hibernation on strain recovery and recovery stress of polyurethane shape memory polymer foam. Smart Materials and Structures, 10(2): 321-25] studied the shape memory functionality of a polyurethane (PU) based SMP foam by performing the conventional thermomechanical programming cycle and recommended those PU based foams be used in foldable space vehicles and quick molding devices. On the other hand, Huang et. al [Huang W M, Lee C W and Teo H P 2006. Thermomechanical behavior of a polyurethane shape memory polymer foam. Journal of Intelligent Material Systems and Structure, 17: 753-60] studied the influence of cold hibernation on the shape memory properties of PU based SMP foams. They concluded that the cold hibernation process did not affect the shape memory properties in spite of keeping them in a compacted state for a prolonged period.
In the context of thermoset SMPs, the different physical and behavioral characteristics of thermoset SMPs relative to standard thermosets make the applicability or suitability of a component of one type of the thermosets of uncertain relevance to the any. For example, with regular thermosets suitability of an additional component depends on the chemical compatibility, viscosity of the molten thermoplastic, and the concentration gradient, whereas for thermoset SMPs, suitability of an additional component also depends on the ability for diffusion under the recovery force.
Previously, it was shown that the stress-controlled programming and confined shape recovery of a SMP based syntactic foam was able to close impact damage repeatedly, efficiently, and almost autonomously (the only human intervention was by heating) [Li G and John M. Composites Science and Technology 2008; 68: 3337-3343]. In this previous study two key aspects were found with compression programmed shape memory effect to self-close an area of damage: 1) reduction of structure volume during programming and 2) the external confinement of the structure during shape recovery.
These two aspects were found to be important in order to repair damage created in the programmed structure that had a reduced volume due to its shape memory functionality. In order to achieve repair, expansion in volume must be resisted by external confinement, whereupon the material will be pushed towards internal open space such as crack, achieving the self-closing purposes. In these earlier schemes, therefore, compression programming was required.
It is noted that, while it seems that tension programming can also repair internal cracks during free shape recovery (the specimen becomes shorter and thus closes the crack), the free recovery changes the geometry of the structure or compromises the dimensional stability as the structure recovers to its original shape. If confined recovery is used after tension programming (i.e., if the dimensional stability is maintained), the internal crack cannot be closed because the material is not allowed to be pushed towards the internal open space (i.e., the crack). Therefore, compression programming is a better choice, as tension programming may not work. Although tension programming “drawing” of SMPs was also known, tension programming was at odds with the earlier SMP-based healing approaches because any shrinkage of SMP material would exacerbate wounds/damage rather than serving to close any defect, particularly where there was external confinement. It was noted that the repeatability in self-closing (up to 7 cycles) comes from the fact that each round of confined shape recovery served dual purposes: one to self-close internal cracks and the other to complete a new round of compression programming [Li G and John M. Composites Science and Technology 2008; 68: 3337-3343]
This combination of closing and reprogramming in Li and John is achieved because the confined shape recovery came about by heating the foam above the Tg, applying a certain compressive stress to the foam due to confinement, and cooling down below Tg while maintaining the prestrain, which is typical for strain-controlled programming. In other words, strain-controlled compression programming was coupled with confined shape recovery. Therefore, although it may seem as if only one programming was conducted at the very beginning of the repeated impact/closing cycles, each shape recovery actually had one prior programming to supply the energy. The only difference was that the subsequent programmings were automatically performed by being coupled with each confined shape recovery. Therefore, in this disclosure of confined shape recovery, one “nominal” programming led to several cycles of shape recovery. [Li G and John M. Composites Science and Technology 2008; 68: 3337-3343] Of note, the method of Li and John although referred to as “healing” was, as discussed in greater detail below, only a closing of the polymer defect; no molecular scale healing was achieved. The approach of Li and John had disadvantages, it lacked the ability to regain a substantial amount of the original integrity as the two sides of a crack were brought back into contact with each other, but were not reconnected in a “healing” manner one to the other. Although applying external confinement can be a challenge, it may be coupled with architectural design of the composite structures such as 3-D woven fabric reinforced composites [Nji J and Li G. Smart Mater Struct 2010; 19: paper No. 035007] or grid stiffened composites [M. John and G. Li. Self-Healing of Sandwich Structures with Grid Stiffened Shape Memory Polymer Syntactic Foam Core. Smart Materials and Structures, Vol. 19, No. 7, paper number 075013 (12 pages), (July, 2010)], where the important external confinement inherently is applied by the structural cells. However, the unmet need and ongoing challenge is that most of the polymer composite structures used today are conventional thermosetting polymers, which do not have shape memory capability. Therefore, prior to the present invention a challenge has been how to imbue such “non-smart” load-bearing polymers with self-healing capability. Moreover, such existing thermoset composites generally do not utilize particular structural elements that imbue them with “external confinement” properties.
Thus, not merely the healing of polymers, but the ability to self-heal structural damage has been a tremendous interest in the scientific community. With polymer repair, an unmet need exists to effectively repair the internal damage autonomously, repeatedly, efficiently, and at molecular-length scale, such as is possible with biological repair processes. Thus, prior to the disclosure of the present invention, a need in the art has existed for a self-closing mechanism that not only closes macroscopic defects, but which also allows for molecular-scale healing of defects without the need for external shape confinement; in particular there is a need for such healing that can be performed on a repeated basis at or near the site of prior damage.
Fiber reinforced polymer (FRP) composite materials have been widely used in various engineering sectors and commercial goods. These include, communication devices, energy storage and transportation, and in commercial goods such as bicycles, sport equipment, etc. These uses derive primarily due to their high specific strength/stiffness and corrosion resistance of such composites.
For example, fiber reinforced polymer (FRP) composite materials have been widely used, including in communication devices (satellites, gyroscopes, etc), energy storage and transportation (such as pipelines, pressure vessels, offshore oil drilling platforms,); and in transportation vehicles (such as aircraft, boats, ships, trains, automobiles, bicycles, etc.); infrastructure (such as in bridges/overpasses in deck panels, concrete bridge columns, girders, and as repair panels); buildings (e.g., cladding for roofs and walls, decks and railings, air flow duct work and ventilation equipment, water handling systems, subterranean uses, seawalls); harbor equipment (e.g., pilings, piers, seawalls and floats); military equipment (such as tanks, military vehicles, armor, etc.); and commercial goods such as safety helmets (sports and transportation), etc. These uses come about primarily due to the high specific strength/stiffness and corrosion resistance of composites.
In addition, ropes and cables made of metal or synthetic fibers have been used as safety ropes (e.g., for mountain climbing, high risk construction, high risk repairs, window washing); such cables are also used as support in cable-stayed bridges. Unfortunately, the current ropes/cables made of polymeric fibers are poor at damping vibration; furthermore, steel wires are used in cable stayed bridge are very heavy.
Although FRPs are typically lighter than metals, owing to the lightweight they are very easily excited (e.g., are subject to vibrations). Without sufficient damping this leads to resonance and structural failure at loads well below ideal design levels. Vibration-induced structural failure is not uncommon. For example, collapse of buildings and bridges during earthquake or hurricane is typical examples of vibration-induced catastrophic structural failure.
For the majority of structural polymers such as epoxy, they are very brittle with very low damping properties. Viscoelastic materials have been widely used for vibration damping. Although adding viscoelastic materials such as rubber particles may improve damping, unfortunately a consequence is that the viscoelastic particles serve as stress concentration centers and result in reduced structural strength and impaired usefulness. Active materials such as shape memory alloy (SMA) have also been used to control vibration. Although SMA is very effective in damping vibration, it is generally both heavy and expensive, and its stiffness does not match that of polymer matrix, and they tend not to be compatible with polymers, limiting SMA application in vibration control.
Moreover, there has been concern over the use and safety of composite structures, such as laminated or sandwich composites, if they become damaged, e.g., if subjected to impact forces. The root cause for these types of damage is the brittleness of polymer matrix and synthetic fibers such as glass fiber or carbon fiber. Even a low velocity impact on laminated or sandwich composites can cause various types of damage such as delamination, face sheet/core debonding, fiber breakage, matrix cracking, and fiber-matrix interfacial debonding. These types of damage are very dangerous because they often cannot be detected visually and may lead to structural failure at loads well below original design levels. Low velocity impact is not uncommon. For example, a drop of a tool during routine maintenance or inspection characterizes a low velocity impact event.
Generally, in fiber reinforced polymer composites, fiber is responsible for carrying the applied load, while polymer matrix is responsible for bonding fiber together, transferring load to the fiber, and protecting the fiber from damage. Consequently, in traditional fiber reinforced polymer composites if fiber is fractured, the load carrying capacity of the composite will be significantly reduced or lost. Unfortunately, fiber fracture is a common failure mode in fiber reinforced polymer composite materials when they are subjected to impact.
Previously, several ways have been explored to enhance the impact tolerance of composites, such as grid stiffened composites and 3-D woven fabric reinforced composites. However, the fundamental reason for fiber fracture has not been overcome and remains an unmet need: the reason is that almost all synthetic fibers such as glass fiber and carbon fiber have high strength but low toughness. As a result, these fibers are good in carrying static load, but inferior in impact tolerance as they can decompensate or rupture when impacted.